Betaines of the aforementioned formula, in which R.sup.1 is an alkyl group with 8 to 14 carbon atoms or the R.sup.4 CO group of which is an acyl group of a fatty acid with 8 to 12 carbon atoms, have increasingly gained in importance in recent years as cleansing agents for the body. They combine excellent cleansing properties with good skin compatibility. In aqueous solution, the betaines form a stable, thick foam, which does not collapse even in the presence of soap.
The synthesis of betaines is described in many patents, of which U.S. Pat. No. 3,225,074 is named as a representative. The appropriate tertiary amine of the general formula II is usually reacted for this purpose with the alkali salt of an .omega.-halogencarboxylic acid, usually, the sodium salt of chloroacetic acid. The reaction preferably takes place in an aqueous medium. The alkali chloride, formed during the reaction, remains in the solution and is not removed.
The betaines of the state of the art are mostly marketed in the form of their 30% by weight aqueous solutions. These betaines are essentially those obtained by the quaternization of fatty alkyl dialkylamines or fatty acid amideamines, the fatty alkyl or fatty acid groups of which, on the average, having 12 to 14 carbon atoms. The longer the chain length of the alkyl group or of the fatty acid mixture used for the synthesis of the betaine, the greater is the increase in the viscosity of the betaine solutions with increasing concentration. It was therefore in the past only possible to prepare aqueous solutions of low concentration, such as 2 to 5% by weight of betaine, from betaines, the R.sup.4 CO acyl group of which is derived from higher molecular weight fatty acids, such as stearic acid.
It is known from the art that the viscosity of betaine solutions can be lowered by aiming for the formation of an aqueous phase with a lamellar structure, a so-called G phase, by the addition of other surfactants. British patent application 2,022,125 A may be mentioned in this context, which relates to a method for producing a concentrated, aqueous, surface-active preparation, which contains, as active component, a mixture of at least two surface active substances, which are not homologous and each of which is present in an amount of at least 5%, based on the mixture. The mixture is capable of forming a liquid G phase. At least one of the surface active substances can be synthesized from a precursor by a reaction in an aqueous solution. Under the reaction conditions, this precursor is a liquid and does not bring about a significant decomposition of the other surface active substance in the mixture. The preparation is formed by converting at least one of the precursors into the corresponding surface active substance in the presence of at least one of the other surface active substance components and carrying out this conversion in the presence of amounts of water, sufficient to keep the reaction mixture in a liquid state and to obtain a final preparation, which is present at least predominantly in the G phase.
This method can best be illustrated by means of an example. In Example 2 of the British patent application 2,022,125 A, 797 g of a 70% solution of a sodium ether lauryl sulfate, which is present in the G phase, is heated together with 442 g of a C.sub.12 /C.sub.14 -alkyldimethylamine and 209 g of chloroacetic acid in 140 g of water, the pH of the mixture being maintained at 7.8.+-.0.2 by the addition of sodium hydroxide solution. The betaine is thus synthesized here in the presence of an alkyl ether sulfate. The product contains 63% of a surface active substance, the ratio by weight of betaine to anionic surfactant being 1:1. The product is liquid and is present in the G phase.
However, this method cannot be employed with betaines, the R.sup.1 alkyl group or the R.sup.4 CO acyl group of which contains 16 or more carbon atoms. With these products in an aqueous medium with anionic surfactants in the concentration ranges given, lamellar structures, which are present in the G phase, are not formed.